The present invention relates to a method for determining physical and/or chemical properties of a medium with reference to at least one first and one second acoustic wave, which each have at least partly propagated through the medium from a transmitter to a receiver and to a device for determining physical and/or chemical properties of a medium.
The medium whose physical and/or chemical properties are to be determined by a generic method preferably is a liquid or a soft material, in particular a highly viscous, dough-like or pasty medium. The acoustic waves utilized for determining the properties for example are ultrasonic waves which are generated by a corresponding transmitter due to a transmit signal.
In a generic method, at least two acoustic waves usually are generated by a transmit signal, which at least partly propagate through the medium along identical or different propagation directions, before they each are received at a receiver located in the respective propagation direction. In a flowing medium, for example, acoustic waves on the one hand are generated in a first propagation direction in flow direction of the medium and on the other hand in a second propagation direction against the flow direction of the medium. From the receive signals generated at the respective receivers, a runtime difference then can be determined and e.g. the (mean) flow velocity of the medium can be inferred therefrom. When absolute runtimes of an acoustic wave from a transmitter to a receiver alternatively or in addition are determined by means of the receive signals, further conclusions about physical and/or chemical properties of the medium can be drawn, such as e.g. about its density, temperature or composition.
From WO 2008/034878 A2 a generic device is known, in which surface acoustic waves are generated, which in a waveguide couple volumetric sound waves into the respective medium. By repeatedly coupling surface waves out at those points at which the volumetric sound wave impinges on a wall bordering the medium, surface acoustic waves in turn are received at a receiver, whose runtimes and runtime differences are characteristic for the medium as well as its physical and/or chemical properties.
In a device described in WO 2008/034878 A2 and in the method realized therewith, the processing of the receive signals generated at the respective receivers for a received acoustic wave—here a surface acoustic wave—is of decisive importance. The determination of a runtime difference or an absolute runtime from the receive signals generated at the receivers by no means is trivial and possibly involves a considerable calculation effort. Depending on the information to be extracted from the receive signals, a variety of different methods is used for signal processing. It is known, for example, to utilize modulated transmit signals, in order to be able to more reliably infer the properties of the medium with reference to the receive signals obtained. For example, there is used a quadrature amplitude modulation, briefly IQ modulation, with which a distinct improvement of the achievable resolution also can regularly be achieved as compared to non-modulated transmit signals.
Further known methods mostly are based on sampled and analog-to-digital converted receive signals, wherein correlation functions, but also the Hilbert and Wavelett transforms are formed.
DE 102 06 134 A1 furthermore discloses a method for determining a flow velocity of a medium by means of ultrasonic waves, which should be resistant to disturbances, such as noise, during signal processing. For this purpose, generated analog receive signals are digitized, a cross correlation is carried out, and a Hilbert conversion is performed, in order to calculate a phase relation between received acoustic waves and therefrom a time difference. A suitable device for processing the receive signals (signal processing device) thus is comparatively complex, however, and hence involves considerable costs.